1. Field of the Invention
The present invention relates generally to a Multi-Input Multi-Output (MIMO) system, and in particular, to an apparatus and method for low-complexity scheduling in a multi-user MIMO system.
2. Description of the Related Art
It is known that a MIMO channel can provide very high transmission efficiency, especially in multi-user environments.
In the multi-user environments, a sum capacity (i.e., a maximum sum of transmission rates) can be achieved by allocating the same resource to a plurality of users simultaneously. The terms ‘user’ and ‘user terminal’ will be interchangeably used herein. In a case of a system using N transmission (hereinafter, “TX”) antennas, the sum capacity (i.e., the theoretical maximum transmission capacity) can be approached by allocating the same resource to up to N users simultaneously in consideration of the complexity of a transmitter. However, the selection of N users for maximization of the transmission capacity is not practically viable because the selection requires a very high complexity.
Several techniques have been proposed to suppress a multi-user interference for the maximization of the transmission capacity. For minimization of the multi-user interference, the above conventional techniques select users, one-by-one, on the basis of channel information fed back from all of the users, which necessitates performing a large amount of computation at a corresponding transmitter. The amount of computation for multi-user selection is determined according to the number of TX antennas and the number of users that will receive data simultaneously. The conventional techniques can reduce the computation amount in linear proportion to the number of users. However, the conventional techniques are not practically viable in real time at a cellular base station because they still require the computation amount corresponding to the fifth-order term of the number of TX antennas.
Hereinafter, a description will be given of a conventional scheduling technique based on a successive projection scheme, assuming that there are K users within the coverage of a serving base station with N TX antennas. The conventional scheduling technique selects a user of a channel that is least correlated with a channel of previously-selected users, i.e., the kth user is selected as Equation (1):
                              u          k                =                                                                              arg                  ⁢                                                                          ⁢                  max                                                                                                                          l                    ≤                    i                    ≤                    K                                    ,                                      i                    ∉                                          S                                              k                        -                        1                                                                                                                          ⁡                      [                                                                                min                                                                                                                                      (                                                                              α                            1                                                    ,                          …                          ⁢                                                                                                          ,                                                      α                                                          k                              -                              1                                                                                                      )                                            ∈                                              R                                                  k                          -                          1                                                                                                                                ⁢                                                                                                                                    ∑                                                  j                          ∈                                                      S                                                          k                              -                              1                                                                                                                          ⁢                                                                        α                          j                                                ⁢                                                  h                          j                                                                                      -                                          h                      i                                                                                        2                                      ]                                              (        1        )            where uk is an index of the kth user, hi is a (1×N) vector of complex numbers that indicates a channel between the ith user and a transmitter, Sk−1={u1, . . . , uk−1} is a set of previously-selected user indexes, α=(α1, . . . , αk−1)ε Rk−1 is a vector that indicates a coefficient for indicating all possible linear combinations from channel information of users selected until the (k−1)th stage, and R is a set of real numbers.
At the kth stage, a user having a channel most orthogonal to a channel of the previously-selected users is selected as Equation (1). Therefore, the user selected at the kth stage has the least interference influence on the previously-selected users.
Assuming that HSk−1 is a matrix corresponding to the accumulation of user channels selected until the (k−1)th stage, Equation (1) can be transformed into a matrix format expressed as Equation (2):
                              u          k                =                                                                              arg                  ⁢                                                                          ⁢                  max                                                                                                                          l                    ≤                    i                    ≤                    K                                    ,                                      i                    ∉                                          S                                              k                        -                        1                                                                                                                          ⁢                                                                                    h                  i                                ⁡                                  [                                                                                                                                          H                                                          S                                                              k                                -                                1                                                                                      *                                                    ⁡                                                      (                                                                                          H                                                                  s                                                                      k                                    -                                    1                                                                                                                              ⁢                                                              H                                                                  S                                                                      k                                    -                                    1                                                                                                  *                                                                                      )                                                                                                    -                          1                                                                    ⁢                                              H                                                  S                                                      k                            -                            1                                                                                                                -                                          I                      N                                                        ]                                                                    2                                              (        2        )            where IN is an (N×N) identity matrix.
The above process is repeated until N users, which will receive data simultaneously, are selected among the K users.
However, a matrix product and an inverse matrix must be computed in order to determine one kth user in the above process, where the size of the inverse matrix increases with an increase in k. Moreover, the computation amount increases because a comparison operation must be performed on all of users that have not yet been selected.